Abstract
The efferent control chain for an upper-limb myoelectric prosthesis can be separated into 3 key areas: signal generation, signal acquisition, and device response. Data were collected from twenty trans-radial myoelectric prosthesis users using their own clinically prescribed devices, to establish the relative impact of these potential control factors on user performance (user functionality and everyday prosthesis usage). By identifying the key factor(s), we can guide future developments to ensure clinical impact. Skill in generating muscle signals was assessed via reaction times and signal tracking. To assess the predictability of signal acquisition, we inspected reaction time spread and undesired hand activations. As a measure of device response, we recorded the electromechanical delay between electrode stimulation and the onset of hand movement. Results suggest abstract measures of skill in controlling muscle signals are poorly correlated with performance. Undesired activations of the hand or incorrect responses were correlated with almost all kinematics and gaze measures suggesting unpredictability is a key factor. Significant correlations were also found between several measures of performance and the electromechanical delay; however, unexpectedly, longer electromechanical delays correlated with better performance. Future research should focus on exploring causes of unpredictability, their relative impacts on performance and interventions to address this.
Highlights
MethodsEach participant attended a 3–4 h testing session, and was provided with breaks throughout including a half hour break mid-way
The findings suggest that future efforts should be concentrated on better understanding why the prosthesis responds unpredictably, and how the electrode interface could be improved to reduce the number of undesired activations of the hand
We noted several significant correlations between the electromechanical delay in the prosthesis and the measures of user performance; these were all in a direction contrary to our hypothesis with longer delays correlating with higher levels of performance
Summary
Each participant attended a 3–4 h testing session, and was provided with breaks throughout including a half hour break mid-way. As the ability to reliably and quickly generate EMG signals in response to prompts will vary across participants, task performance was assessed using the “ideal” skin–electrode interface (Fig. 1) to act as a baseline level of unpredictability. The goniometer data was sampled at 1000 Hz and filtered using a 2nd order double-pass Butterworth filter with a cut-off frequency of 20 Hz. By measuring the time between the start of goniometer recording (synchronised with the switch activating) and the onset of hand opening/closing (angle exceeded 1° above or below the mean resting value calculated from the first 80 ms) it was possible to quantify the delay. In our previous s tudy[17] we found no relationship between our measures of functionality and everyday prosthesis usage, but it is reasonable to assume that more daily wear and increased symmetry of upper-limb activity may be correlated with higher levels of EMG skill, lower unpredictability, and shorter electromechanical delays. To enable readers to further explore these relationships, for correlations τb > 0.3 or τb < − 0.3 (regardless of p-value), we have provided scatter plots as supplementary material
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